Anion specificity influencing morphology in catanionic surfactant mixtures with an excess of cationic surfactant

In previous studies we reported on an ion-specific micelle-to-vesicle transition, when salts are added to a catanionic micellar solution composed of sodium dodecylcarboxylate (sodium laurate, SL) and dodecyltrimethylammonium bromide (DTAB), with an excess of SL. In the present paper, we illustrate the ion specificity, when DTAB is in excess in the same system. In this case, no transition to vesicles is observed, but an elongation of micelles, when salts are added. The counterion binding and increase in aggregate size were monitored by mass spectrometry, dynamic light scattering measurements, and cryo-transmission microscopy. The mechanism is argued by employing the ability of the counterion to dehydrate the surfactant headgroup. This hypothesis was recently confirmed by MD and ab initio calculations and in phase with Collins' concept of matching water affinities.